UPDATED to include details of the Justice Department’s suit: California Gov. Jerry Brown today signed a measure that restores Obama-era open-Internet rules in the state, in a direct rebuke to  the Trump Administration’s rollback of these regulations. The Justice Department responded with a lawsuit seeking to prevent the law from taking effect.

The California legislature approved a bill that is regarded as having the strongest net-neutrality provisions in the nation. The legislation forbids Internet providers from blocking legal websites, intentionally slowing down Internet traffic or demanding fees for faster service.

The Democratic governor, in signing the measure into law, sets California on a collision course with the FCC, which earlier this year dismantled the landmark net-neutrality rules that ensured equal access to the Internet.

It didn’t take long for the Trump Administration to respond to a direct challenge to its regulatory authority. The Justice Department filed suit in federal district court in California later Sunday, arguing that the state was attempting to subvert the federal government’s rules for regulating broadband communications.

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It argues that the federal government — not the states — have the right to regulate interstate communications.

“Under the Constitution, states do not regulate interstate commerce—the federal government does,” said Attorney General Jeff Sessions in a statement. “Once again the California legislature has enacted an extreme and illegal state law attempting to frustrate federal policy.”

The FCC argues, in its court filing, that it expressly pre-empted state or local legislation that would be “inconsistent” with its deregulatory approach. Some legal scholars say the FCC’s decision to deregulate an industry doesn’t give it the authority to prevent states from imposing their own regulations.

FCC chairman Ajit Pai telegraphed that a fight was brewing in a recent speech in Maine, suggested California’s net-neutrality law would be “illegal” and vowed a fight to protect the new “light touch,” telecom-friendly regulations he introduced.

Pai applauded the DOJ’s decision to challenge California’s new law as both illegal and harmful to consumers.

“The law prohibits many free-data plans, which allow consumers to stream video, music, and the like exempt from any data limits,” Pai said in a statement. “They have proven enormously popular in the marketplace, especially among lower-income Americans. But notwithstanding the consumer benefits, this state law bans them.”

Pai is referring to the law’s ban on a practice called “zero rating.” These plans allow a wireless subscriber to watch certain content without it counting against their monthly data cap. That’s a clever way for companies like, say, AT&T, to advantage its own service (say, the DirecTV Now service) over a competitor’s.

The DOJ suit sets up yet another legal clash that pits California against the Trump Administration. State Attorney General Xavier Becerra has filed a flurry of suits against the federal government challenging it on across a range of issues including immigration, the environment, education and health care.

Becerra issued a statement, suggesting he was up for another fight.

“While the Trump Administration continues to ignore the millions of Americans who voiced strong support for net neutrality rules, California — home to countless start-ups, tech giants and nearly 40 million consumers — will not allow a handful of power brokers to dictate sources for information or the speed at which websites load,” Becerra said. “We remain deeply committed to protecting freedom of expression, innovation and fairness.”

Open-Internet advocates have been urging Brown to sign, saying he needs to “stand up to big telecom companies” that nearly derailed the bill this summer.

“Despite their army of lobbyists and millions spent lining the pockets of legislators, these companies continue to lose ground in the face of overwhelming cross-partisan opposition to their greedy attacks on our Internet freedom,” said Evan Greer, deputy director of Fight for the Future, a digital rights group that has been advocating for the law’s passage. “When all is said and done, Comcast, Verizon, and AT&T are going to wish they’d never picked a fight with Internet over net neutrality. Other states should follow California’s lead, and Congress should pass the joint resolution to reverse the FCC’s resoundingly unpopular repeal.”

The bill’s author, Sen. Scott Wiener (D-San Francisco), took a brief victory lap, declaring this a historic day for California. Then he issued a defiant statement, taking issue with Sessions’ view that the state lacks authority to  protect the open Internet.

“Weve been down this road before: when Trump and Sessions sued California and claimed we lacked the power to protect immigrants. California fought Trump and Sessions on their immigration lawsuit — California won — and California will fight this lawsuit as well,” Weiner said. “I have complete confidence that Attorney General Xavier Becerra will do a great job defending this law.

Jonathan Spalter, president and CEO of USTelecom, said members of the trade group supports enforceable net-neutrality protections for every American, but that California’s law was not the way to accomplish that goal.

“This bill is neither the way to get there, nor will it help advance the promise and potential of California’s innovation DNA,” Spalter said. “Rather than 50 states stepping in with their own conflicting open internet solutions, we need Congress to step up with a national framework for the whole internet ecosystem and resolve this issue once and for all.”

Under California law, if Brown had taken no action, the net-neutrality rules would take effect in January.